There are a variety of chemical reactions that may be accelerated under conditions of a temperature substantially above ground surface ambient temperature and a pressure substantially above atmospheric pressure. A major part of the reactor apparatus heretofore provided for carrying out the various chemical reactions at higher temperatures and pressures typically require high pressure liquid pumps, high pressure, high temperature heat exchangers and pressure vessels with rotating seals and considerable land surface area.
One chemical reaction for which the method and apparatus of the present invention is particularly suitable is for the direct wet oxidation of materials in a waste stream and particularly the direct wet oxidation of sewage sludge. The everincreasing amounts of sewage sludge which are, of course, directly related to the population increases are an increasing concern particularly of municipalities.
The Zimpro method, Barber-Coleman process, and Navy shipboard processors are examples of current methods used to effect direct wet oxidation of sewage sludge and all involve placing the waste in a high temperature, high pressure reactor at substantially ground surface level. Air is pumped into the reactor vessel and heat is externally applied. Constant mechanical stirring is required to mix oxygen, a reactant, into the liquid and to remove carbon dioxide, a product of the reaction. Even though recuperative heat exchangers are employed, large amounts of energy are consumed in order to maintain the reaction temperature. There are also a number of obvious environmental and excessive energy problems presently associated with handling sewage sludge. Due to the nature of the sewage wastes and high pressures necessary, the equipment, operating, and maintenance costs in such methods are exceedingly high. Also, due to the high temperature and pressure, there is an attendant safety hazard.
Present practices frequently involve the transportation of the dried sewage sludge many miles to a land fill point. The land filling of dried sewage sludge presently in use has a number of obvious disadvantages. Some attempt has been made to carry out accelerated chemical reactions below the ground surface level using the increased pressures provided by a hydrostatic column of liquid or fluid. In this connection particular attention is directed to U.S. Pat. Nos. 3,449,247 to Bauer, 3,606,999 to Lawless, and 3,853,759 to Titmus.
Bauer discloses the concentration of the wastes or the addition of combustible refuse to sustain the wet oxidation process and injects air at the bottom of the hydrostatic column. Injection of air at the bottom of course requires greater pressures, larger compressors, and increases the overall costs and energy requirements. No means for controlling the temperature of the fluid in the reaction zone is suggested.
The disclosure made by Lawless is again the introduction of air at the bottom of the reactor. The bubbles introduced by Lawless are of a minute size with a large aggregate surface area. Such a bubble configuration increases the pressure differential and limits the reaction rate by restricting the flow of reactants and products across the boundary layer, thereby requiring a considerable consumption of pumping power to move the liquid through the system as compared to a system that employs enlarged bubbles. Excess heat is removed from the apparatus by dilution of the incoming feed or by withdrawal of vapor. Dilution of the incoming feed decreases the total treatment capacity and increases the unit treatment costs. Lawless also discloses the necessity of additional support for the center flow passage and provides multiple floats to accomplish this objective.
The Titmus disclosure limits any oxidation reactions to the amount of oxygen present in the material. No additional air or oxygen is added to the waste stream. Such a treatment method for sewage sludge is generally undesirable since pyrolysis of the waste results in the production of free carbon and other undesirable products. This disclosure contemplates the necessary, continuous supply of heat energy to effect the reactions. Furthermore, the rate of heat energy input controls the flow and provides a thermal head which causes the effluent to flow from the reactor. Placement of the steam line in the effluent causes an energy loss to the liquid and increases energy and operating costs.
Accordingly, it is an object of the present invention to provide a novel method and apparatus for accelerating chemical reactions at elevated temperatures and pressures.
Another object of the present invention is to provide a method and apparatus for effecting chemical reactions characterized by minimum initial capital costs, a relatively high overall thermodynamic efficiency, safety, durability, and an overall simplicity of construction and operation with a minimum land area requirement.
A further object of the present invention is to provide a novel below ground surface method and apparatus for enhancing chemical reactions at temperatures and pressures considerably above atmospheric ambient temperatures and atmospheric pressures without the necessity of the customary high pressure liquid pumps, high pressure, high temperature heat exchangers and pressure vessels with rotating seals required when the reaction takes place at the ground surface level.
Yet another object of the present invention is to provide a novel and improved method and apparatus for accelerating chemical reactions characterized by a reaction that takes place below the ground surface level, with all of the control in the area of the ground surface level, and with only minimum power requirements for pumping liquids and/or gases therethrough.
Still a further object of the present invention is to provide a novel and improved method and apparatus for accelerating chemical reactions that is particularly effective in converting a stream of waste materials such as sewage sludge to purified water and useful heat energy from the reacted waste stream.
Yet a further object of the present invention is to provide a novel and improved method and apparatus for accelerating chemical reactions characterized by the use of a hydraulic U-tube that extends a substantial depth below the earth's surface with a continuous fluid flow throughput to achieve the necessary high pressures due to the hydrostatic head of a column of liquid formed in the U-tube and the use of enlarged bubbles to introduce a gas to enhance chemical reaction with close control of all the flow rate, pressure, temperature and volume parameters necessary to carry out the process with maximum efficiency.
A further object of the present invention is to provide a novel method and apparatus that has reaction rates that are higher than those in comparable reactors using mechanical stirrers.